This invention relates generally to aircraft structures and more particularly to a laminar flow control (LFC) structure and a method for manufacturing the same.
It is commonly understood that the boundary layer flow over the surfaces of an aircraft in flight is laminar over the upstream portions of the surfaces, but becomes turbulent over the downstream portions. It has been recognized for some time that the aerodynamic efficiency of airplanes could be increased significantly if the boundary layer flow could be maintained in a laminar condition over more of the aerodynamic surfaces, principally because the skin friction drag resulting from laminar flow is much less than the drag resulting from turbulent flow. It has also been known for some time that flow can be kept laminar by drawing a portion of the boundary layer through openings in the aerodynamic surfaces to prevent boundary layer build-up and consequent transition to turbulent flow. This technique has been tested and proven both in the laboratory and on full-scale aircraft.
In spite of the successes of these test programs the use of laminar flow control for production aircraft has generally been considered impractical for a number of reasons. First, the manufacture of structures incorporating laminar flow control according to earlier teachings was both complicated and prohibitively expensive. Next, most LFC wing designs involved unacceptable reductions in the available fuel volume and unacceptable increases in weight. In addition, inspection of the structures was difficult or impossible and field repair was complicated and expensive.
Basically, a laminar flow control system requires many small openings in the aerodynamic surfaces through which boundary layer flow can be drawn and a sytsem of ducts located within the structure for carrying the boundary layer flow away. Most embodiments found in the prior art used some sort of double skin arrangement wherein boundary layer flow was drawn through slots machined directly in the outer skin and passed into ducts formed between the inner and outer skins.
Typical of such designs is the structure described in U.S. Pat. No. 3,117,751, to K. H. Rogers, et al. The patent discloses a structure consisting of an outer skin formed from a honeycomb sandwich panel which incorporates a slotted outer sheet, a honeycomb sandwich inner panel, and a number of spanwise stiffeners which are bonded to the outer panel and riveted to the inner panel.
Primary ducts are formed by the walls of the inner and outer skin and the bonded stiffeners, and small tributary ducts are attached to the inner surface of the outer skin panels. While this embodiment was successfully used in a full-scale research aircraft, it exhibited a number of deficiencies. The weight of the wing structure was significantly increased due to the inefficient use of the inner and outer panels as bending material. Manufacturing costs were significantly greater than for a conventional wing structure. Repair of the panels was difficult and costly and structural inspection required the removal of both the inner and outer panels. The design required the use of blind fasteners to attach the stiffeners to the inner skin panels, which provided undesirable fatigue characteristics and the possibility of fuel leakage into the duct areas. Unfortunately, no means was provided to detect such leakage. Further, the design also involved an estimated 17% reduction in volume within the wing available for fuel.
Another laminar flow control apparatus relevant to this invention is described in U.S. Pat. No. 3,521,837 to Hermann Papst. His patent appears to disclose an airfoil having a wing skin in which recesses and throttle holes have been machined into the upper surface. In one embodiment slots through which boundary layer air can be drawn are formed between the adjacent edges of two parallel strips of metal mounted on opposite sides of the upper portion of the recesses. In another, a plurality of slots are cut in a single strip of metal which is mounted in the upper recess. As with many other known laminar flow control devices this device would increase manufacturing costs and would be difficult to repair in the field. It would also be vulnerable to damage during subsequent manufacturing operations and would not insure smoothness of the upper skin surface after assembly.
In spite of the shortcomings of the various designs found in the prior art, recent emphasis on more efficient and economical aircraft designs has prompted a serious reconsideration of the use of laminar flow control devices. Accordingly, one objective of this invention is to provide for a method of manufacturing a laminar flow control structure which is more economical and practical than those formerly known. Another objective of this invention is to provide for a laminar flow control structure which an acceptable weight penalty and a minimal reduction in available fuel volume as compared to conventional structures. It is a further objective of this invention to provide for a laminar flow control structure which can be easily repaired in the field and which permits a structural inspection of skins and stiffeners. Finally, a further objective of this invention is to provide for a laminar flow control structure in which the smoothness of the external aerodynamic surfaces is assured to be within allowable tolerances.